[Tolk] Support syntax tensorVar.0 and tupleVar.0

It works both for reading and writing:
> var t = (1, 2);
> t.0;      // 1
> t.0 = 5;
> t;        // (5, 2)

It also works for typed/untyped tuples, producing INDEX and SETINDEX.

Global tensors and tuples works. Nesting `t.0.1.2` works. `mutate` works.
Even mixing tuples inside tensors inside a global for writing works.

tolk-tester/tests/invalid-assign-2.tolk

@@ -0,0 +1,11 @@
+fun incThree(mutate a: int, mutate b: int, mutate c: int) {}
+
+fun main() {
+    var c = [[[1, 2]]];
+    incThree(mutate c.0.0.0, mutate c.0.0.1, mutate c.0.0.0);
+}
+
+/**
+@compilation_should_fail
+@stderr one variable modified twice inside the same expression
+*/